The H3+ + H2 isotopic system: origin of deuterium astrochemistry

Abstract

Dense cold molecular clouds reckoned to be stellar nurseries are the scene of an extreme molecular deuteration. Despite the cosmic D/H ratio of 10^{-5}, molecular species in prestellar cores are observed to contain nearly as much deuterium as hydrogen. This astonishing deuterium enrichment promoted by low temperatures is the work of H3+. It is the key species which unlocks the deuterium from its HD reservoir via reactions like H3+ + HD -> H2D+ + H_2 and drags it further to other species in successive reactions. For this reason, the H3+ + H2 isotopic system is outstandingly critical for the astrochemistry of cold environments. However, its understanding is yet incomplete and insufficient. This thesis thus focuses on the H3+ + H2 isotopic system from a theoretical, experimental and astronomical point of view giving a particular look into the role of nuclear spins. As a first step, the stringent nuclear spin selection rules in associative, dissociative and reactive collisions are investigated. This purely theoretical study zooms into the details of the nuclear spin wavefunctions and shows that their permutation symmetry representation is necessary and sufficient, contrary to their angular momentum representation. Additionally, a new deterministic interpretation of nuclear spins in chemical reactions is proposed. Based on these considerations, a complete set of state-to-state rate coefficients for all H3+ + H2 isotopic variants is calculated using a microcanonical model leaned on phase space theory. An experimental study is conducted in parallel with a 22-pole ion trap apparatus in order to inspect the influences of temperature and H2 ortho-to-para ratio. The good overall agreement between experimental and theoretical results supports the validity and utility of the calculated set of rate coefficients. Furthermore, the potentiality of the 22-pole ion trap apparatus is explored via the Laser Induced Reaction (LIR) technique applied to our system of interest. High resolution overtone, combination and fundamental vibrational spectroscopy of H2D+ and D2H+ is thereby achieved with cw-OPO and diode lasers. Finally, astronomical implications are inferred on an observational basis through the case of the prestellar core L183 using simple chemical models which account for the ortho, meta and para characters of the H3+ and H2 isotopologues and rely on the rate coefficients derived in this thesis. Above all, the results show that the non-thermal ortho-to-para ratio of H2 is a serious limiting factor for the enhancement of deuterium fractionations. It is a first-class parameter for the astrochemistry of very cold interstellar medium.